Not to be confused with the halo headband, the halo eyeliner effect is when there's a very thin layer of some color other outside the eyeliner on the bottom (and sometimes on top, too). The key here is that the line is very very thin, so it looks like the color is almost radiating from the darker eyeliner itself—you know, like the way a halo would glow. It's a way to add a touch of color to your eye looks without getting too far from the usual "smoky" category and into "funky."